Vehicles having hybrid powertrains may be used where the engine is periodically shut-off during vehicle operation to improve overall fuel economy. In one example approach, the engine may be pre-positioned to obtain a desired engine position for future starts. In one example, the desired pre-position is with at least one cylinder near TDC (end of compression/beginning of expansion) for a 4-cylinder, even firing, 4-cycle engine with cam actuated valves.
However, the inventor herein has recognized at least the following issues with such an approach. For example, positioning a cylinder near TDC (end of compression/beginning of expansion) often results in another cylinder being positioned near its TDC (end of exhaust/beginning of intake) and having both intake and exhaust valves at least partly open. This condition creates a path from the intake, through the cylinder, to the exhaust. In other words, there is communication between the intake and exhaust manifolds. This condition can result in oxygen migration from the intake manifold, cylinder, or exhaust to the catalyst during engine-off conditions, thereby saturating the catalyst with excess oxygen. The migration can be exacerbated by emission testing conditions, which can draw exhaust flow even when the engine is off. This flow of air through the catalyst can also cool the catalyst to below its optimal operating temperature.
At least some of the above issues may be addressed by a method for controlling an engine that operates with negative valve overlap between an intake valve and an exhaust valve during at least some conditions, the method comprising: providing an engine stop position with at least one cylinder near top dead center of piston position; and adjusting at least one of the intake valve and the exhaust valve to position the at least one of the intake and exhaust valves of said at least one cylinder so that the at least one of said intake and exhaust valves is substantially closed at said engine stop position.
In this way, it may be possible to provide improved restarting of the engine, yet reduce the potential for migration of oxygen from the intake and/or engine cylinders to the exhaust catalyst system.